Optical Power Meter and Optical Loss Tester

An optical power meter, when used with a calibrated light source, measures optical loss by comparing transmitted and received power levels along a fiber link.

Understanding Optical Loss Testing

Optical loss testing evaluates the attenuation of light in a fiber optic cable, which is critical for ensuring signal quality and network performance. The measurement is not a direct attenuation reading but rather a comparison of optical power levels at the input and output of the fiber. This is typically done using an Optical Loss Test Set (OLTS), which combines an Optical Light Source (OLS) and an Optical Power Meter (OPM) . The OLS injects a known optical signal at a specific wavelength into the fiber, while the OPM measures the power received at the far end. The difference between transmitted and received power, expressed in dB, represents the fiber's optical loss .

Equipment and Setup

1. Optical Power Meter (OPM): A calibrated device with a photodiode sensor (Silicon, Germanium, or InGaAs) that measures light power in dBm. It must be set to the correct wavelength to ensure accuracy .
2. Optical Light Source (OLS): Provides a stable, known optical signal at the test wavelength. Some advanced OLS units can encode multiple wavelengths for simultaneous testing .
3. Connectors and Patch Cords: Clean and properly mated connectors are essential to avoid measurement errors. Use reference patch cords to establish baseline power levels .

Step-by-Step Measurement Procedure

1. Warm up the OPM: Turn on the meter and allow it to stabilize for a minute or two .
2. Set the wavelength: Match the OPM to the wavelength of the OLS (commonly 850 nm, 1310 nm, or 1550 nm) for accurate readings .
3. Clean connectors: Ensure all fiber end-faces and adapters are free of dust or debris .
4. Connect the fiber: Attach the OLS to the fiber input and the OPM to the fiber output. For end-to-end loss, measure the power at the far end of the installed fiber .
5. Record readings: The OPM displays the received power in dBm. Subtract this from the transmitted power to calculate optical loss in dB .

Practical Considerations

• Calibration: Ensure the OPM is calibrated according to a traceable standard to maintain accuracy .
• Multiple wavelengths: Test at all relevant wavelengths to account for wavelength-dependent loss.
• Pass/Fail Limits: Many modern OPMs and OLTS kits provide automatic evaluation against user-defined limits, simplifying field testing .
• Bidirectional Testing: For higher accuracy, measure loss in both directions and average the results, especially in multi-fiber links . Using an optical power meter in combination with a light source provides a reliable and standardized method to quantify optical loss, ensuring fiber links meet performance specifications and minimizing signal degradation in communication networks.